Obstacle traversing wheelchair

ABSTRACT

The present invention provides a curb-climbing wheelchair having pivotal arm assemblies. The pivotal arm assemblies include at least one front caster and have the ability to rotate or pivot. The pivot arms rotate or pivot in response to moments generated by accelerating or decelerating the wheelchair thereby raising or lowering the front caster(s) through the rotational movement of the pivot arms. By raising or lowering the front caster(s) in such a manner, curb-like obstacles can be traversed in a low-impact manner.

CROSS REFERENCES TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 09/698,481, filed on Oct. 27, 2000, now U.S. Pat. No. 6,554,086, andtitled “Obstacle Traversing Wheelchair.”

FIELD OF THE INVENTION

The invention relates generally to wheelchairs, and more particularly,to a wheelchair having pivotal assemblies for traversing obstacles suchas curbs and the like.

BACKGROUND OF THE INVENTION

Wheelchairs are an important means of transportation for a significantportion of society. Whether manual or powered, wheelchairs provide animportant degree of independence for those they assist. However, thisdegree of independence can be limited if the wheelchair is required totraverse obstacles such as, for example, curbs that are commonly presentat sidewalks, driveways, and other paved surface interfaces.

In this regard, most wheelchairs have front and rear casters tostabilize the chair from tipping forward or backward and to ensure thatthe drive wheels are always in contact with the ground. One suchwheelchair is disclosed in U.S. Pat. No. 5,435,404 to Garin. On suchwheelchairs, the caster wheels are typically much smaller than thedriving wheels and located both forward and rear of the drive wheels.Though this configuration provided the wheelchair with greaterstability, it made it difficult for such wheelchairs to climb overobstacles such as, for example, curbs or the like, because the frontcasters could not be driven over the obstacle due to their small sizeand constant contact with the ground.

U.S. Pat. No. 5,964,473 to Degonda et al. describes a wheelchair havingfront and rear casters similar to Garin and a pair of additional forwardlift wheels. The lift wheels are positioned off the ground and slightlyforward of the front caster. Configured as such, the lift wheels firstengage a curb and cause the wheelchair to tip backwards. As thewheelchair tips backwards, the front caster raises off the ground to aheight so that it either clears the curb or can be driven over the curb.

While Degonda et al. addressed the need of managing a front caster whiletraversing an obstacle such as a curb, Degonda et al. is disadvantageousin that additional wheels (i.e., lift wheels) must be added to thewheelchair. Hence, it is desirable to provide a wheelchair that does notrequire additional lift wheels or other similar type mechanisms to raisea front caster off the ground to a height so that the caster eitherclears an obstacle or can be driven over the obstacle.

SUMMARY OF THE INVENTION

According to a general embodiment of the present invention, a wheelchairfor traversing obstacles is provided. The wheelchair includes, forexample, a frame, a pivoting assembly, and a drive assembly. Thepivoting assembly has at least one pivot arm having a first portion,second portion and third portion. The first portion is pivotally coupledto the frame. The second portion has at least one caster attachedthereto. The drive assembly is coupled to the third portion of the pivotarm. In operation, the pivot arm pivots in response to the forcesgenerated by the drive assembly, which is coupled to the pivot arm. Asused herein, when two objects are described as being coupled orattached, it is applicants' intention to include both direct couplingand/or attachment between the described components and indirect couplingand/or attachment between the described components such as through oneor more intermediary components.

According to a more specific embodiment of the present invention, awheelchair for traversing obstacles having for example, a frame and aseat for seating a passenger are provided. Pivotally coupled to theframe are a pair of pivot arms. Each pivot arm has a first distalportion, a second distal portion, and a pivotal connection between thefirst and second distal portions for pivotally coupling the pivot arm tothe frame. A motor is coupled to the first distal portion and a frontcaster is coupled to the second distal portion of each pivot arm. Adrive wheel is coupled to each motor for translating the motor'srotational energy to the ground. At least one rear caster is coupled tothe frame to provide for rear stability. By accelerating the wheelchairforward, the drive wheels generate a moment causing each pivot arm topivot or rotate thereby raising the front casters to a height sufficientto traverse the obstacle.

According to another aspect of the present invention, a secondembodiment of a obstacle traversing wheelchair is provided. The secondembodiment includes, for example, a frame and a seat for seating apassenger. Pivotally coupled to the frame are a pair of pivot armshaving casters connected thereto. Each pivot arm has a first distalportion and a second distal portion that acts as a pivotal connectioncoupling the arm to the frame. A motor is pivotally coupled to eachpivot arm at a location between the first and second distal portions.The pivotal coupling between the motor and the pivot arm is furtherinfluenced by a resilient member providing suspension between the motorand pivot arm. The motor is preferably a gearless, brushless,direct-drive motor although brush-type motors with transmissions canalso be used. A front resilient assembly is coupled to the frame and themotor's pivotal connection to the pivot arm so as to provide a constantresilient force between the frame, the motor's pivotal connection, andthe arm.

According to another aspect of the present invention, a method oftraversing one or more obstacles is provided. The method includes, forexample, accelerating a wheelchair toward the one or more obstacles and,through such accelerating, causing a raising of one or more frontcasters by pivoting an arm that is coupled to the one or more frontcasters so that the one or more front casters are raised to a heightsufficient for the one or more front casters to traverse the obstacle.The step of pivoting the arm coupled to the one or more front castersincludes, for example, the step of generating a moment associated withthe pivot arm causing the pivot arm to rotate in the direction ofraising the one or more front casters. The height by which the frontcasters must be raised to traverse an obstacle varies from raising thefront casters to a height where their axles are just above the height ofthe obstacle to raising the front casters to a height where the casters'lower extremities are above the height of the obstacle. In the casewhere the front casters are raised to a height where their axles arejust above the height of the obstacle, the wheelchair engages the frontcasters with the obstacle and drives the front casters there over.

According to another aspect of the present invention, a method ofdescending curb-like obstacles is also provided. In particular, thepresent invention lowers the front casters over a curb onto the newlower elevation when descending to provide forward stability for awheelchair while the drive wheels and rear caster(s) are still on highercurb elevation. As the drive wheels continue over the curb and contactthe new lower elevation forward stability is still maintained by virtueof the front casters while the rear caster is still on the higher curbelevation.

It is, therefore, an advantage of the present invention to provide acost-efficient wheelchair that can traverse one or more curb-likeobstacles.

It is, therefore, another advantage of the present invention to providea mid-wheel drive wheelchair with pivotable front caster assemblies.

It is, therefore, a further advantage of the present invention toprovide a torque-based method of raising the front casters of awheelchair for traversing curb-like obstacles.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings which are incorporated in and constitute apart of the specification, embodiments of the invention are illustrated,which, together with a general description of the invention given above,and the detailed description given below, serve to example theprinciples of this invention.

FIGS. 1 and 2A are front and rear perspective views, respectively, of afirst embodiment of a wheelchair of the present invention.

FIG. 2B is a front perspective view of an alternative embodiment of thewheelchair of FIGS. 1 and 2A having a stabilizing torsion element.

FIG. 3 is an exploded perspective view of certain components of thefirst embodiment.

FIGS. 4A, 4B, and 4C are illustrations showing the forces acting on thewheelchair of the first embodiment in the static, accelerating anddecelerating mode of operation.

FIGS. 5A, 5B, 5C, 5D, and 5E sequentially illustrate the curb-climbingoperation of the first embodiment.

FIGS. 6A, 6B, 6C, and 6D sequentially illustrate the curb descendingoperation of the first embodiment.

FIGS. 7 and 8 are front and rear perspective views, respectively, of asecond embodiment of a wheelchair of the present invention.

FIG. 9A is an exploded perspective view of certain components of thesecond embodiment.

FIG. 9B is an enlarged view of a portion of FIG. 9A showing an assembleddrive wheel and caster arrangement.

FIGS. 10A, 10B, and 10C are illustrations showing the forces acting onthe wheelchair of the second embodiment in the static, accelerating anddecelerating mode of operation.

FIGS. 11A, 11B, 11C, 11D, and 11E sequentially illustrate thecurb-climbing operation of the second embodiment.

FIGS. 12A, 12B, 12C, 12D, and 12E correspond to enlarge portions ofFIGS. 11A, 11B, 11C, 11D, and 11E, respectively, particularly showingthe sequential range of motion of a front resilient assembly of thepresent invention.

FIGS. 13A, 13B, 13C, and 13D sequentially illustrate the curb-descendingoperation of the second embodiment.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

Referring now to the drawings, and more particularly to FIGS. 1 and 2A,perspective views of a wheelchair 100 of the present invention areshown. The wheelchair 100 has a pair of drive wheels 102 and 104, frontcasters 106 and 108, rear caster 110, and front riggings 112 and 114.The front riggings 112 and 114 include footrests 116 and 118 forsupporting the feet of a passenger. The front riggings 112 and 114 arepreferably mounted so as to be able to swing away from the shown centerposition to the sides of wheelchair 100. Additionally, footrests 116 and118 can swing from the shown horizontal-down position to a vertical-upposition thereby providing relatively unobstructed access to the frontof wheelchair 100.

The wheelchair 100 further includes a chair 120 having a seat portion122 and a back portion 124 for comfortably seating a passenger. Chair120 is adjustably mounted to frame 142 so as to be able to move forwardand backward on frame 142, thereby adjusting the passenger's weightdistribution and center of gravity relative to the wheelchair. In themost preferred embodiment, chair 120 should be positioned such that asubstantial portion of the wheelchair's weight when loaded with apassenger is generally above and evenly distributed between drive wheels102 and 104. For example, the preferred weight distribution ofwheelchair 100 when loaded with a passenger should be between 80% to 95%(or higher) on drive wheels 102 and 104. The remainder of the weightbeing distributed between the front and rear casters. Armrests 126 and128 are also provided for resting the arms of a passenger or assisting apassenger in seating and unseating from chair 120.

The wheelchair 100 is preferably powered by one or more batteries 130,which reside beneath the chair 120 and in-between drive wheels 102 and104. A pair of drive motors 136 and 138 and gearboxes are used to powerdrive wheels 102 and 104. The motors and their associated transmissionsor gearboxes (if any) forming a drive assembly. A control system andcontroller (not shown) interface batteries 130 to the drive motors 136and 138 so as to allow a passenger to control the operation of thewheelchair 100. Such operation includes directing the wheelchair'sacceleration, deceleration, velocity, braking, direction of travel, etc.

Front casters 106 and 108 are attached to pivot arms 132 and 134,respectively. Rear caster 110 is attached to rear caster arm 140. Whileonly one rear caster is shown, it should be understood that in thealternative two rear casters can also be provided. As will be describedin more detail, pivot arms 132 and 134 are pivotally coupled to frame142 for curb climbing and descending, while rear caster arm 140 isrigidly coupled to frame 142.

Springs 144 and 146 are coupled to the arms 132 and 134 and the frame142. More specifically, the coupling to arms 132 and 134 is preferablyvia attachment to the housings of motors 136 and 138, respectively. Thecoupling to the frame 142 is via attachment to seat back 124. Soconfigured, each spring provides a spring force urging the motorhousings upward and the seat 120 or the rearward portion of frame 142downward.

FIG. 2B is a partial front perspective view of wheelchair 100 showing atorsion bar 200 of the present invention. Beyond a certain range ofmotion, torsion bar 200 ensures that arms 132 and 134 influence eachother. In this regard, torsion bar 200 has a torsion section 206 andstem sections 208 and 218. Torsion bar 200 is preferably made by takinga stock of spring steel and performing two bends in the stock to formtorsion section 206 and stem sections 208 and 210. As shown in FIG. 2B,arms 132 and 134 have attached thereto first and second torsion mountingelements 202 and 204. Each torsion mounting element includes asemi-circular groove therein for accepting a stem section of the torsionbar 200. The torsion bar 200 is held in place within torsion mountingelements 202 and 204 via forced fit within the semi-circular grooves. Inoperation, arm 132 or 134 is free to independently move (i.e., raise orlower) a limited distance before it influences the other arm via torsionbar 200. More specifically, once the torsion limit of torsion bar 200 isexceeded, it behaves as a substantially rigid member translating anyfurther motion of one arm to the other arm.

The suspension and drive components of wheelchair 100 are furtherillustrated in the exploded prospective view of FIG. 3. Morespecifically, pivot arm 132 has a base member 306 and an angled member302 extending therefrom. The distal end of angled member 302 includes afront swivel assembly 304 that interfaces with a front caster 106. Basemember 306 has attached thereto a mounting plate 308 for mounting drivemotor 136 and gearbox assembly 309. Drive motor 136 is coupled to pivotarm 132 through gearbox assembly 309 and mounting plate 308. The gearboxassembly 309 interfaces drive motor 136 to drive wheel 102, which ismounted on drive axle 311. The gearbox assembly 309 is preferablyattached to mounting plate 308 with screws or bolts and mounting plate308 is preferably welded to base member 306.

Pivot arm 132 has a pivot mounting structure between base member 306 andangled member 302. The pivot mounting structure includes brackets 310and 312 and sleeve 314. Brackets 310 and 312 are preferably welded tobase member 306 and sleeve 314 is preferably welded to brackets 310 and312, as shown. A low-friction sleeve 316 is provided for sleeve 314 andis inserted therein.

Frame 142 has longitudinal side members 318 and 320 and cross-bracemembers 322 and 324. Cross-brace members 322 and 324 are preferablywelded to longitudinal side members 318 and 320, as shown. A pair offrame brackets 326 and 328 are preferably welded to longitudinal sidemember 318. The frame brackets 326 and 328 are spaced apart such thatsleeve 314 can be inserted there between and further include guide holesor apertures such that a pin or bolt 330 can be inserted through bracket326, sleeve 314, and bracket 328. In this manner, pivot arm 132 and itsattachments can pivot around bolt 330 and are pivotally mounted to frame142. Pivot arm 134 is similarly constructed and mounted to frame 142.

Referring now to FIGS. 4A through 4C, free body diagrams illustratingvarious centers of gravity and the forces acting on wheelchair 100 willnow be described. In particular, FIG. 4A is a free body diagramillustrating the forces acting on wheelchair 100 when the wheelchair isin static equilibrium. The various forces shown include F_(p), F_(b),F_(s), F_(fc), F_(rc), and F_(w). More specifically, F_(p) is the forcerepresenting gravity acting on the center of gravity of a person C_(gp)sitting in wheelchair 100. Similarly, F_(b) is the force representinggravity acting on the center of gravity of the batteries C_(gb) used topower wheelchair 100. Resilient member or spring 144 introduces aresilient force F_(s) acting on pivot arm 132 through its connection tothe housing of drive motor 136. A second resilient member or spring 146(see FIG. 3) provides a similar force on pivot arm 134. Rear caster 110has a force F_(rc) acting on its point of contact with the ground. Frontcaster 106 has a force F_(fc) acting on its point of contact with theground. Front caster 108 (not shown in FIG. 4A) has a similar forceacting on it as well. Drive wheel 102 has force F_(w) acting on itspoint of contact with the ground and drive wheel 104 also has a similarforce acting thereon.

In wheelchair 100, the center of gravity of a person C_(gp) sitting inthe wheelchair is preferably located behind a vertical centerline 402through pivotal connection P. Similarly, the center of gravity of thebatteries C_(gb) is located behind the vertical centerline 402. Asalready described, it is possible to obtain between approximately 80% to95% weight distribution on drive wheels 102 and 104, with the remainderof the weight being distributed between the front casters 106 and 108and the rear caster 110. As will be explained in more detail, such anarrangement facilitates the raising and lowering of the front casters106 and 108 during acceleration and deceleration of the wheelchair 100.

Under static equilibrium such as, for example, when the chair is at restor not accelerating or decelerating as shown in FIG. 4A, the netrotational moment around pivotal connection P and pivot arms 132 and 134is zero (0) (i.e., ΣF_(n)r_(n)=0, where F is a force acting at adistance r from the pivotal connection P and n is the number of forcesacting on the wheelchair). Hence, pivot arms 132 and 134 do not tend torotate or pivot.

In FIG. 4B, wheelchair 100 is shown accelerating. The forces are thesame as those of FIG. 4A, except that an acceleration force F_(a) isacting on drive wheel 102. A similar force acts on drive wheel 104. Whenthe moment generated by the acceleration force F_(a) exceeds the momentgenerated by spring force F_(s), pivot arm 132 will begin to rotate orpivot such that front caster 106 begins to rise. As the moment generatedby the acceleration force F_(a) continues to increase over the momentgenerated by spring force F_(s), the pivot arm 132 increasingly rotatesor pivots thereby increasingly raising front caster 106 until themaximum rotation or pivot has been achieved. The maximum rotation orpivot is achieved when pivot arm 132 makes direct contact with frame 142or indirect contact such as through, for example, a pivot stop attachedto frame 142. Pivot arm 134 and front caster 108 behave in a similarfashion.

Hence, as the wheelchair 100 accelerates forward and the moment createdby accelerating force F_(a) increases over the moment created by springforce F_(s), pivot arms 132 and 134 begin to rotate or pivot therebyraising front casters 106 and 108 off the ground. As described, it ispreferable that front casters 106 and 108 rise between 1 and 6 inchesand most preferably between 1 and 4 inches off the ground so as to beable to traverse a curb or other obstacle of the same or similar height.

Referring now to FIG. 4C, a free body diagram illustrating the forcesacting on wheelchair 100 when the wheelchair is decelerating is shown.The forces are the same as those of FIG. 4A, except that a decelerationforce F_(d) is acting on drive wheel 102 instead of an accelerationforce F_(a). A similar force acts on drive wheel 104. The momentgenerated by the deceleration force F_(d) causes pivot arm 132 to rotatein the same direction as the moment generated by spring force F_(s),i.e., clockwise as shown. If front caster 106 is not contacting theground, this pivot arm rotation causes front caster 106 to lower untilit makes contact with the ground. If front caster 106 is alreadycontacting the ground, then no further movement of front caster 106 ispossible. Hence, when wheelchair 100 decelerates, front caster 106 isurged towards the ground. Pivot arm 134 and front caster 108 behave in asimilar manner.

The spring force F_(s) can be used to control the amount of accelerationand deceleration that is required before pivot arm 132 pivots and raisesor lowers front caster 106. For example, a strong or weak spring forcewould require a stronger or weaker acceleration and deceleration beforepivot arm 132 pivots and raises or lowers front caster 106,respectively. The exact value of the spring force F_(s) depends ondesigner preferences and overall wheelchair performance requirements foracceleration and deceleration. For example, the spring force F_(s) mustbe strong enough to keep chair 120 and the passenger from tippingforward due to inertia when the wheelchair is decelerating. It shouldalso be noted that, in conjunction with the spring force F_(s), thecenter of gravity of the person C_(gp) sitting in the wheelchair can bemodified. For example, the center of gravity C_(gp) may be moved furtherrearward from vertical centerline 402 by moving chair 120 rearward alongframe 142 with or without adjusting the magnitude of the spring forceF_(s). Moreover, the position of pivotal connection P may be moved alongthe length of pivot arms 132 and 134 thereby changing the ratio ofdistances between the pivotal connection P and the motor driveassemblies and casters 106 and 108 thereby resulting changing thedynamics of the pivot arms and wheelchair. Hence, a combination offeatures can be varied to control the pivoting of pivot arms 132 and 132and the raising and lowering of front casters 106 and 108.

Referring now to FIGS. 5A through 5E, the curb-climbing capability ofwheelchair 100 will now be described. In FIG. 5A, the wheelchair 100approaches a curb 502 of approximately 2 to 4 inches in height. Thewheelchair 100 is positioned so that front casters 106 and 108 areapproximately 6 inches from the curb 502. Alternatively, wheelchair 100can be driven directly to curb 502 such that front casters 106 and 108bump against curb 502 and are driven thereunto, provided the height ofcurb 502 is less than the axle height of front casters 106 and 108 (notshown).

Nevertheless, in FIG. 5B from preferably a standstill position, drivemotors 136 and 138 are “torqued” so as to cause pivot arms 132 and 134to pivot about, for example, pin or bolt 330 and raise front casters 106and 108 off the ground. The torquing of drive motors 136 and 138 refersto the process by which drive motors 136 and 138 are directed toinstantaneously produce a large amount of torque so that theacceleration force F_(a) creates a moment greater than the momentgenerated by spring force F_(s). Such a process is accomplished by thewheelchair's passenger directing the wheelchair to accelerate rapidlyfrom the standstill position. For example, a passenger can push hard andfast on the wheelchair's directional accelerator controller (not shown)thereby directing the wheelchair to accelerate forward as fast aspossible. As shown in FIG. 5B and as described in connection with FIGS.4A-4C, such “torquing” causes pivot arms 132 and 134 to pivot about pin330 thereby causing front casters 106 and 108 to rise. During torquing,the wheelchair 100 accelerates forward toward the curb 502 with thefront casters 106 and 108 in the raised position.

In FIG. 5C, front casters 106 and 108 have passed over curb 502. Asfront casters 106 and 108 pass over or ride on top of curb 502, drivewheels 102 and 104 come into physical contact with the rising edge ofcurb 502. Due to the drive wheels' relatively large size compared to theheight of curb 502, the drive wheels 102 and 104 are capable of engagingcurb 502 and driving there over—thereby raising the wheelchair 100 overcurb 502 and onto a new elevation. Once raised, the front casters 106and 108 are lowered as the inertial forces of the passenger and batteryapproach zero. These inertial forces approach zero when wheelchair 100either decelerates such as, for example, by engaging curb 502 or byaccelerating wheelchair 100 to its maximum speed (under a given loading)at which point the acceleration approaches zero and wheelchair 100approaches the state of dynamic equilibrium. Either scenario causespivot arms 132 and 134 to lower front casters 106 and 108 onto the newelevation.

FIG. 5D shows wheelchair 100 after the drive wheels 102 and 104 havedriven over curb 502 and onto the new elevation with front casters 106and 108 lowered. Rear caster 110 still contacts the previous lowerelevation. By such contact, rear caster 110 provides rearward stabilitypreventing wheelchair 100 from tipping backwards as the wheelchairclimbs the curb 502. FIG. 5E illustrates wheelchair 100 after rearcaster 110 has engaged and surmounted curb 502.

Hence, the present invention provides a feature by which the frontcasters of a wheelchair can be raised and lowered when the wheelchairmust climb or surmount a curb or obstacle. By raising the front castersto an appropriate position, whether completely clear of the curb orobstacle height or partially clear thereof, the wheelchair's drivewheels can, in effect, drive the wheelchair over the curb or obstacle.

Referring now to FIGS. 6A through 6D, the curb-descending capability ofwheelchair 100 will now be described. Referring now particularly to FIG.6A, wheelchair 100 slowly approaches a curb 602, which represents a dropin elevation. In FIG. 6B, front casters 106 and 108 have gone over curb602 and are in contact with the new lower elevation. As front casters106 and 108 go over the curb or obstacle 602, they are urged downwardtoward the new lower elevation by the force generated by springs 144 and146. This results in very little impact or feeling of loss of stabilityto the wheelchair passenger because the wheelchair 100 stayssubstantially level as the front casters 106 and 108 drop over curb 602to the new lower elevation.

In FIG. 6C, drive wheels 102 and 104 have gone over curb 602 and are incontact with the new lower elevation. As drive wheels 102 and 104 goover curb 602, wheelchair 100 is prevented from tipping forward bysprings 144 and 146 and front casters 106 and 108. More specifically,springs 144 and 146 urge the back of seat 120 rearward to counter anyforward tipping tendency that the wheelchair may exhibit. In addition orin the alternative, an electromechanical stop or spring dampener can beenergized by sensing inertial forces, angle of the wheelchair frame, orcurrent to or from the drive motors, which would prevent the wheelchairfrom tipping forward (not shown).

In FIG. 6D, rear caster 110 has gone over curb 602 and contacts the newlower elevation. As rear caster 110 drops down over curb or obstacle602, very little impact or instability is experienced by the wheelchairpassenger because most of the wheelchair's weight (including passengerweight) is supported by drive wheels 102 and 104, which are already onthe new lower elevation. Hence, as rear caster 110 goes over curb 602and contacts the new lower elevation, the wheelchair passengerexperiences a low-impact transition between elevations.

Therefore, wheelchair 100 provides a stable, low-impact structure andmethod for climbing or descending over curb-like obstacles. In climbingcurb-like obstacles, wheelchair 100 raises the front casters to a heightsufficient for the front casters to go over the curb-like obstacle andallow the wheelchair's drive wheels to engage the obstacle. The rearcaster provides rearward stability during such curb-climbing. Indescending curb-like obstacles, wheelchair 100 lowers the front castersover the obstacle to provide forward stability as the drive wheels driveover the obstacle. The resilient members or springs provide rearwardstability by urging the rear of the wheelchair's seat downward tocounter any forward tipping tendency that the wheelchair may exhibitwhen descending a curb or obstacle. Additionally, chair or seat 120 canbe moved rearward or tilted backward to increase wheelchair stabilitywhen descending a curb or obstacle.

Referring now to FIGS. 7 and 8, a second embodiment of a curb-climbingwheelchair 700 of the present invention is shown. The wheelchair 700 hasa pair of drive wheels 702 and 704, front casters 706 and 708, rearcaster 710, and front riggings 712 and 714. As in wheelchair 100, thefront riggings 712 and 714 include footrests 716 and 718 for supportingthe feet of a passenger. The front riggings 712 and 714 are preferablymounted so as to be able to swing away from the shown center position tothe sides of the wheelchair. Additionally, footrests 716 and 718 canswing from the shown horizontal-down position to a vertical-up positionthereby providing relatively unobstructed access to the front of thewheelchair.

The wheelchair 700 further includes a chair 720 having a seat portion722 and a back portion 724 for comfortably seating a passenger. Chair720 is adjustably mounted to frame 742 (see FIG. 8) so as to be able tomove forward and backward on frame 742, thereby adjusting thepassenger's weight distribution and center of gravity relative to thewheelchair. As in wheelchair 100, chair 720 is preferably positionedsuch that a substantial portion of the wheelchair's weight when loadedwith a passenger is evenly distributed between drive wheels 702 and 704.For example, the preferred weight distribution of wheelchair 700 whenloaded with a passenger should be between 80% to 95% (or higher) ondrive wheels 702 and 704. The remainder of the weight being distributedbetween the rear and front casters. Armrests 726 and 728 are alsoprovided for resting the arms of a passenger or assisting a passenger inseating and unseating from chair 720.

The wheelchair 700 is preferably powered by one or more batteries 730,which reside beneath the chair 720 and in-between drive wheels 702 and704. A pair of drive motors 736 and 738 (see FIG. 8) are used to powerdrive wheels 702 and 704. Drive motors 736 and 738 are preferablybrushless, gearless, direct-drive motors with their rotors eitherinternal or external to their stators. Drive motors 736 and 738 alsoeach include a fail-safe braking mechanism that includes a manualrelease mechanism (not shown). A control system and controller (notshown) interface batteries 730 to drive motors 736 and 738 so as toallow a passenger to control the operation of the wheelchair 700. Suchoperation includes directing the wheelchair's acceleration,deceleration, velocity, braking, direction of travel, etc.

Front casters 706 and 708 are attached to pivot arms 732 and 734,respectively. Rear caster 710 is attached to rear caster arms 740A and740B (see FIG. 8). While only one rear caster is shown, it should beunderstood that in the alternative two or more rear casters can also beprovided. As will be described in more detail, pivot arms 732 and 734are pivotally coupled to frame 742 for curb-climbing and descending,while rear caster arm 740A and 740B are rigidly coupled to frame 742.

The suspension and drive components of wheelchair 700 are furtherillustrated in the exploded prospective view of FIG. 9A. Morespecifically, pivot arm 732 has a base portion 906, an angled portion902 extending therefrom, and a motor mount bracket 910. The distal endof angled portion 902 includes a front swivel assembly 904 thatinterfaces with front caster 706. Base portion 706 has a portionincluding a hole 905 for pivot pin 922 and associated sleeve fittings.

The suspension further includes a coupling plate 914 for interfacingfront resilient assembly 931 to pivot arm 732. Coupling plate 914 ispreferably rigidly affixed to pivot arm 732 via rigid tubular connection916. Coupling plate 914 has a mounting bracket 918 configured to receivea pivot pin for interfacing to front resilient assembly 931. Configuredas such, pivot arm 732 and coupling plate move in unison about pivot pinor bolt 922 subject to the forces and moments generated by frontresilient assembly 931 and motor 736. Additionally, the suspension canfurther include a torsion member (not shown) between pivot arms 732 and734 similar to the arrangement shown in FIG. 2B.

A resilient suspension member such as spring 920 extends between and isconnected at its opposite ends to pivot arm 732 to a motor mount 908.Motor mount 908 has a pivot connection 912 that pivotally couples motormount bracket 910 to pivot arm 732 and coupling plate 914 via a pivotpin. More specifically, motor mount 908 is pivotally received in a spacebetween motor mount bracket 910 and coupling plate 914. Motor mount 908further includes holes for fastening motor 136 thereto. Configured assuch, motor 736 is pivotally coupled to pivot arm 732, which is itselfpivotally coupled to frame 742.

Referring now to FIGS. 9A and 10A, front resilient assembly 931 has aspring 938 that is indirectly coupled to frame 742 and coupling plate914 via arcuate pivot brackets 932 and 934 and horizontal pivot bracket936. Arcuate pivot brackets 932 and 934 are generally curved and haveholes in their distal portions. The holes are used for securing arcuatepivot brackets 932 and 934 to frame mounting bracket 940 and tohorizontal pivot bracket 936 via screws or pins. Spring 938 is coupledto the lower portions of arcuate pivot brackets 932 and 934 proximate toframe mounting bracket 940 and to one of a plurality of points shownbetween the distal portions of horizontal pivot bracket 936.

In this regard, horizontal pivot bracket 936 has a first distal portionhaving a pivot hole for interfacing with coupling plate 914 and, moreparticular, spring mounting bracket 918. The other distal portion ofhorizontal pivot bracket 936 has a plurality of mounting holes thatallow for the mounting of arcuate pivot brackets 932 and 934 in variouspositions. So configured front resilient assembly 931 is similar infunction to springs 144 and 146 of wheelchair 100. However, theconfiguration of linkages 932, 934, and 936 and spring 938 of frontresilient assembly 931 provide for a constant spring force over therange of pivoting of pivot arm 732.

FIGS. 11A through 11E and 12A through 12D illustrate the response of thefront resilient assembly 931 linkages with respect to wheelchair 700climbing and descending a curb-like obstacle.

Still referring to FIG. 9A, frame 742 includes longitudinal side members924 and 926 and cross-brace members 928 and 930. Pivot arm 732 ispivotally mounted to side members 926 through pivot arm base member 906and pin 922. Motor 736 is pivotally mounted to pivot arm 732 throughmotor mount 908 and its pivot assembly 912. Since motor 736 is pivotalwith respect to pivot arm 732, spring 920 provides a degree ofsuspension between the two pivotal components. Additionally, since pivotarm 732 pivots with respect to frame 742, spring 938 and associatedvertical and horizontal pivot brackets 934, 936, and 938, respectively,urge pivot arm 732 such that front caster 706 is urged downward towardthe riding surface. This is similar in functionality to spring 144 ofwheelchair 100.

FIG. 9B is an enlarged view of portion 942 of FIG. 9A. Morespecifically, portion 942 shows pivot arm 734 and its associatedcomponents, which are similarly configured to pivot arm 732 and itsassociated assemblies, in their assembled positions on frame 742.

Referring now to FIGS. 10A through 10C, free body diagrams illustratingvarious centers of gravity and the forces acting on wheelchair 700 willnow be described. In particular, FIG. 10A is a free body diagramillustrating the forces acting on wheelchair 700 when the wheelchair isin static equilibrium. The various forces shown include F_(p), F_(b),F_(s), F_(fc), F_(rc), and F_(w). As described in FIGS. 4A-4C, F_(p) isthe force representing gravity acting on the center of gravity of aperson C_(gp) sitting in wheelchair 700. Similarly, F_(b) is the forcerepresenting gravity acting on the center of gravity of the batteriesC_(gb) used to power wheelchair 100. Spring 944 introduces a force F_(s)acting on pivot arm 732. Spring 938 (see FIG. 9A) provides a similarforce on pivot arm 732. Rear caster 710 has a force F_(rc) acting on itspoint of contact with the ground. Front caster 708 has a force F_(fc)acting on its point of contact with the ground. Front caster 706 (seeFIG. 9A) has a similar force acting on it as well. Drive wheel 704 hasforce F_(w) acting on its point of contact with the ground and drivewheel 702 (see FIG. 9A) also has a similar force acting thereon.

In wheelchair 700, the center of gravity C_(gp) of a person sitting inthe chair is preferably located just behind a vertical centerline 1002through pivotal connection P. Similarly, the center of gravity C_(gb) ofthe batteries is located behind the vertical centerline 1002. As alreadydescribed, it is possible to obtain between approximately 80% to 95%weight distribution on drive wheels 702 and 704, with the remainder ofthe weight being distributed between the front casters 706 and 708 andthe rear caster 710. As will be explained in more detail, such anarrangement facilitates the raising and lowering of the front casters706 and 708 during acceleration and deceleration of the wheelchair 700.

Under static equilibrium such as, for example, when the chair is at restor not accelerating or decelerating as shown in FIG. 10A, the netrotational moment around pivotal connection P and pivot arms 732 and 734is zero (0) (i.e., ΣF_(n)r_(n)=0, where F is a force acting at adistance r from the pivotal connection P and n is the number of forcesacting on the wheelchair). Hence, pivot arms 732 and 734 do not tend torotate or pivot.

In FIG. 10B, wheelchair 700 is shown accelerating. The forces are thesame as those of FIG. 10A, except that an acceleration force F_(a) isacting on drive wheel 704. A similar force acts on drive wheel 702. Whenthe moment generated by the acceleration force F_(a) exceeds the momentgenerated by spring force F_(s), pivot arm 734 will begin to rotate orpivot such that front caster 708 begins to rise. As the moment generatedby the acceleration force F_(a) continues to increase over the momentgenerated by spring force F_(s), pivot arm 734 increasingly rotates orpivots thereby increasingly raising front caster 708 until the maximumrotation or pivot has been achieved. The maximum rotation or pivot isachieved when pivot arm 734 makes direct contact with frame 742 orindirect contact such as through, for example, a pivot stop attached toframe 742. Pivot arm 734 and front caster 708 behave in a similarfashion.

Hence, as the wheelchair 700 accelerates forward and the moment createdby accelerating force F_(a) increases over the moment created by springforce F_(s), pivot arms 732 and 734 being to rotate or pivot therebyraising front casters 706 and 708 off the ground. As described, it ispreferable that front casters 706 and 708 rise between 1 and 6 inchesoff the ground so as to be able to overcome a curb or other obstacle ofthe same or similar height.

Referring now to FIG. 10C, a free body diagram illustrating the forcesacting on wheelchair 700 when the wheelchair is decelerating is shown.The forces are the same as those of FIG. 10A, except that a decelerationforce F_(d) is acting on drive wheel 702 instead of an acceleratingforce F_(a). A similar force acts on drive wheel 702. The momentgenerated by the deceleration force F_(d) causes pivot arm 734 to rotatein the same direction as the moment generated by spring force F_(s),i.e., clockwise as shown. If front caster 708 is not contacting theground, this pivot arm rotation causes front caster 708 to lower untilit makes contact with the ground. If front caster 708 is alreadycontacting the ground, then no further movement of front caster 708 ispossible. Hence, when wheelchair 700 decelerates, front caster 708 isurged clockwise or towards the ground. Pivot arm 732 and front caster706 behave in a similar manner.

As with wheelchair 100, the spring force F_(s) can be used to controlthe amount of acceleration and deceleration that is required beforepivot arm 734 pivots and raises or lowers front caster 708. For example,a strong or weak spring force would require a stronger or weakeracceleration and deceleration before pivot arm 734 pivots and raises orlowers front caster 708, respectively. The exact value of the springforce F_(s) depends on designer preferences and overall wheelchairperformance requirements for acceleration and deceleration. For example,the spring force F_(s) must be strong enough to keep chair 720 and thepassenger from tipping forward due to inertia when the wheelchair isdecelerating. Additionally, because horizontal pivot bracket 936 has aplurality of mounting holes (see FIG. 9A, for example) for mountingvertical pivot brackets 932 and 934, the amount of spring force F_(s)applied to the pivot arms can also be controlled by the appropriatechoice of mounting for such brackets. It should also be noted that,either alone or in conjunction with the spring force F_(s) and thevertical and horizontal pivot bracket configuration, the center ofgravity of the person C_(gp) sitting in the wheelchair can be modified.For example, the center of gravity C_(gp) may be moved further rearwardfrom vertical centerline 1002 with or without adjusting the magnitude ofthe spring force F_(s). Hence, a combination of features can be variedto control the pivoting of pivot arms 732 and 732 and the raising andlowering of front casters 706 and 708.

Referring now to FIGS. 11A through 11E, the curb-climbing capability ofwheelchair 700 will now be described. In FIG. 11A, the wheelchair 700approaches a curb 1102 of approximately 3 to 6 inches in height. Thewheelchair 700 is positioned so that front casters 706 and 708 areapproximately 6 inches from the curb 1102. Alternatively, wheelchair 700can be driven directly to curb 1102 such that front casters 706 and 708bump against curb 1102 and are driven thereunto, provided the height ofcurb 1102 is less than the axle height of front casters 706 and 708 (notshown).

Nevertheless, in FIG. 11B from preferably a standstill position, drivemotors 736 and 738 are “torqued” so as to cause pivot arms 732 and 734to pivot about, for example, pin or bolt 922 and raise front casters 706and 708 off the ground. As described earlier, the torquing of drivemotors 736 and 738 refers to the process by which drive motors 736 and738 are directed to instantaneously produce a large amount of torque sothat the acceleration force F_(a) creates a moment greater than themoment generated by spring force F_(s). Such a process is accomplishedby the wheelchair's passenger directing the wheelchair to acceleraterapidly from the standstill position. For example, a passenger can pushhard and fast on the wheelchair's directional accelerator controller(not shown) thereby directing the wheelchair to accelerate forward asfast as possible. As shown in FIG. 11B and as described in connectionwith FIGS. 10A-10C, such “torquing” causes pivot arms 732 and 734 topivot about pin 922 thereby causing front casters 706 and 708 to rise.During torquing, the wheelchair 700 accelerates forward toward the curb1102 with the front casters 706 and 708 in the raised position.

In FIG. 11C, front casters 706 and 708 have passed over curb 1102. Asfront casters 706 and 708 pass over or ride on top of curb 1102, drivewheels 702 and 704 come into physical contact with the rising edge ofcurb 1102. Due to the drive wheels' relatively large size compared tothe height of curb 1102, the drive wheels 702 and 704 are capable ofengaging curb 1102 and driving there over—thereby raising the wheelchair700 over curb 1102 and onto a new elevation. As drive wheels 702 and 704engage curb 1102, suspension spring 920 cushions the impact of thetransition. Once raised, the front casters 706 and 708 are lowered asthe inertial forces of the passenger and battery approach zero. Theseinertial forces approach zero when wheelchair 700 either deceleratessuch as, for example, by engaging curb 1102 or by acceleratingwheelchair 700 to its maximum speed (under a given loading) at whichpoint the acceleration approaches zero and wheelchair 700 approaches thestate of dynamic equilibrium. Either scenario causes pivot arms 732 and734 to lower front casters 706 and 708 onto the new elevation.

FIG. 11D shows wheelchair 700 after the drive wheels 702 and 704 havedriven over curb 1102 and onto the new elevation with front casters 706and 708 lowered. Rear caster 710 still contacts the previous lowerelevation. By such contact, rear caster 710 provides rearward stabilitypreventing wheelchair 700 from tipping backwards as the wheelchairclimbs the curb or obstacle 1102. FIG. 11E illustrates wheelchair 700after rear caster 710 has engaged and surmounted curb or obstacle 1102.FIGS. 12A, 12B, 12C, 12D, and 12E correspond to enlarge portions ofFIGS. 11A, 11B, 11C, 11D, and 11E, respectively, particularly showingthe orientation and range of motion experienced by front resilientassembly 931 as the wheelchair climbs a curb.

Hence, the embodiment of wheelchair 700 provides a feature by which thefront casters of a wheelchair can be raised and lowered when thewheelchair must climb or surmount a curb or obstacle. By raising thefront casters to an appropriate position, whether completely clear ofthe curb or obstacle height or partially clear thereof, the wheelchair'sdrive wheels can, in effect, drive the wheelchair over the curb orobstacle.

Referring now to FIGS. 13A through 13D, the curb descending capabilityof wheelchair 700 will now be described. Referring now particularly toFIG. 13A, wheelchair 700 slowly approaches a curb 1302, which representsa drop in elevation. In FIG. 13B, front casters 706 and 708 have goneover curb 1302 and are in contact with the new lower elevation. As frontcasters 706 and 708 go over curb 1302, they are urged downward towardthe new lower elevation by the force generated by springs 938 and 944.This results in very little impact or feeling of loss of stability tothe wheelchair passenger because the wheelchair 700 stays substantiallylevel as the front casters 706 and 708 drop over curb 1302 to the newlower elevation.

In FIG. 13C, drive wheels 702 and 704 have gone over curb 1302 and arein contact with the new lower elevation. As drive wheels 702 and 704 goover curb or obstacle 1302, suspension springs such as spring 920cushion the impact of such a transition. Also as drive wheels 702 and704 go over curb 1302, wheelchair 700 is prevented from tipping forwardby springs 938 and 944 and front casters 706 and 708. More specifically,springs 938 and 944 urge the front of frame 742, through frame mountingbracket 940 (see FIGS. 9 and 10), upward to counter any forward tippingtendency that the wheelchair may exhibit.

In FIG. 13D, rear caster 710 has gone over curb 1302 and contacts thenew lower elevation. As rear caster 710 drops down over curb 1302, verylittle impact or instability is experienced by the wheelchair passengerbecause most of the wheelchair's weight (including passenger weight) issupported by drive wheels 702 and 704, which are already on the newlower elevation. Hence, as rear caster 710 goes over curb 1302 andcontacts the new lower elevation, the wheelchair passenger experiences alow-impact transition between elevations.

Therefore, wheelchair 700 provides a stable, low-impact structure andmethod for climbing or descending over curb-like obstacles. In climbingcurb-like obstacles, wheelchair 700 raises the front casters to a heightsufficient for the front casters to go over the curb-like obstacle andallow the wheelchair's drive wheels to engage the obstacle. The rearcaster provides rearward stability during such curb-climbing. Indescending curb-like obstacles, wheelchair 700 lowers the front castersover the obstacle to provide forward stability as the drive wheels driveover the obstacle. Suspension springs associated with the drive wheelsprovide for low-impact transitions for the passenger between elevationsrepresenting curbs or obstacles. Springs associated with the frontcasters provide forward stability by urging the front of thewheelchair's frame upward to counter any forward tipping tendency thatthe wheelchair may exhibit when descending a curb or obstacle.

While the present invention has been illustrated by the description ofembodiments thereof, and while the embodiments have been described inconsiderable detail, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. For example, the pivot arms can be made from aplurality of components having differing geometry, the wheelchair may ormay not include spring forces acting on the pivot arms, the inventioncan be applied to rear-wheel and front-wheel drive wheelchairs,elastomeric resilient members can be used instead of or in combinationwith springs, electrically adjustable spring tension devices can beincluded with the springs, etc. Therefore, the invention, in its broaderaspects, is not limited to the specific details, the representativeapparatus, and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of the applicant's general inventive concept.

1. A wheelchair for traversing obstacles comprising: (a) frame; (b) apivoting assembly having at least one pivot arm, said at least one pivotarm comprising (i) a first portion pivotally coupled to said frame; and(ii) a second portion having at least one front caster attached theretofor contacting a supporting surface of the wheelchair; (c) a driveassembly coupled to a third portion of said at least one pivot arm; (d)a rear caster coupled to said frame; and wherein said at least one pivotarm pivots in response to a moment generated by the drive assemblythereby raising said at least one front caster from said supportingsurface to assist said wheelchair in traversing said obstacles; andwherein said drive assembly is pivotally coupled to said third portionof said at least one pivot arm.
 2. A wheelchair for traversing obstaclescomprising: (a) a frame; (b) a pivoting assembly having at least onepivot arm, said at least one pivot arm comprising (i) a first portionpivotally coupled to said frame; and (ii) a second portion having atleast one front caster attached thereto for contacting a supportingsurface of the wheelchair; (c) a drive assembly coupled to a thirdportion of said at least one pivot arm; (d) a rear caster coupled tosaid frame; and wherein said at least one pivot arm pivots in responseto a moment generated by the drive assembly thereby raising said atleast one front caster from said supporting surface to assist saidwheelchair in traversing said obstacles; wherein said pivot assemblyfurther comprises a resilient member coupled to said at least one pivotarm and said frame; and wherein said resilient member is coupled to saidat least one pivot arm via a pivoting horizontal bracket.
 3. Thewheelchair of claim 2 wherein said resilient member is further coupledto said frame via at least one pivoting arcuate bracket.
 4. A wheelchairfor traversing obstacles comprising: (a) a frame; (b) a pivotingassembly having at least one pivot arm, said at least one pivot armcomprising (i) a first portion pivotally coupled to said frame; and (ii)a second portion having at least one front caster attached thereto forcontacting a supporting surface of the wheelchair; (c) a drive assemblycoupled to a third portion of said at least one pivot arm; (d) a rearcaster coupled to said frame; (e) a suspension device disposed betweensaid at least one pivot arm and said drive assembly; and wherein said atleast one pivot arm pivots in response to a moment generated by thedrive assembly thereby raising said at least one front caster from saidsupporting surface to assist said wheelchair in traversing saidobstacles.
 5. The wheelchair of claim 4 wherein said suspension devicecomprises a spring.
 6. A wheelchair for traversing obstacles comprising:(a) a frame; (b) a pivoting assembly having at least one pivot arm, saidat least one pivot arm comprising (i) a first portion pivotally coupledto said frame; and (ii) a second portion having at least one frontcaster attached thereto for contacting a supporting surface of saidwheelchair; (c) a drive assembly pivotally coupled to a third portion ofsaid at least one pivot arm, said third portion comprising a positionbetween said first and second portions; and (d) at least one rear castercoupled to the frame.
 7. The wheelchair of claim 6 wherein said at leastone pivot arm pivots in response to a moment generated by the driveassembly thereby raising said at least one front caster from saidsupporting surface to assist said wheelchair in traversing saidobstacles.
 8. The wheelchair of claim 6 further comprising a resilientassembly coupled to said frame and said at least one pivot arm.
 9. Thewheelchair of claim 8 wherein said resilient assembly comprises: (a) atleast one horizontal pivot bracket; (b) at least one arcuate pivotbracket coupled to said frame and said at least one horizontal pivotbracket; and (c) at least one resilient member coupled to said at leastone horizontal pivot bracket and said at least one arcuate pivotbracket.
 10. The wheelchair of claim 6 further comprising a suspensionmember coupled between said at least one pivot arm and said driveassembly.
 11. The wheelchair of claim 6 wherein said drive assemblycomprises a brushless motor.
 12. The wheelchair of claim 6 furthercomprising a second pivot arm and a tension bar coupled to the pivotarms for limiting the independent movement of each pivot arm.
 13. Awheelchair comprising: (a) a frame; (b) a pivoting assembly having atleast one pivot arm, said at least one pivot arm comprising: (i) a firstportion pivotally coupled to said frame; and (ii) a second portionhaving at least one front castor attached thereto for contacting asupporting surface of said wheelchair; (c) a drive assembly pivotallycoupled to said at least one pivot arm; and (d) at least one rear castorcoupled to the frame.
 14. The wheelchair of claim 13 further comprisinga second pivot arm and a tension bar coupled to the pivot arms forlimiting the independent movement of each pivot arm.
 15. A wheelchaircomprising: (a) a frame means having first and second side members; (b)a suspension coupled to the frame means; said suspension comprising: (i)an arm pivotally coupled to one of the side members and having a castorconnected thereto; (ii) a drive assembly pivotally coupled to the arm;and (iii) a spring positioned between said pivot arm and said driveassembly.
 16. The wheelchair of claim 15 further comprising a second armpivotally coupled to one of the side members and a tension bar coupledto the arms for limiting the independent movement of each arm.
 17. Thewheelchair of claim 15 further comprising a second arm pivotally coupledto one of the side members and a resilient means coupled to the firstand second pivot arms for allowing restricted motion respectivelybetween the first and second pivot arms.
 18. A wheelchair comprising:(a) a frame means having first and second side members: (b) a suspensioncoupled to the frame means; said suspension comprising: (i) first andsecond pivot means respectively coupled to the first and second sidemembers; and (ii) third and fourth pivot means respectively coupled tothe first and second pivot means (c) first and second drive meansrespectively coupled to the third and fourth pivot means; (d) frontcastor means coupled to the first and second pivot means; (e) rearcastor means coupled to the frame means; and (f) a suspension springmeans for limiting the movement of the third and fourth pivot meansrelative the first and second pivot means; and wherein said first andsecond pivot means pivot in response to a moment generated by the drivemeans thereby raising the front caster means from said supportingsurface to assist said wheelchair in traversing an obstacle.
 19. Thewheelchair of claim 18 further comprising a torsion means limiting themovement of the first and second pivot means relative to one another.20. The wheelchair of claim 18 further comprising a spring means forurging the first and second pivot means in a first direction.
 21. Thewheelchair of claim 18 wherein the drive means comprises a motor. 22.The wheelchair of claim 18 wherein the drive means further comprises atransmission.